As is well known in the art, the production of ethylene by the pyrolytic cracking of hydrocarbons normally involves a series of treatment steps which can be generally described in the following manner. A hydrocarbon feedstock such as gas oil or naphtha is introduced into a high severity cracking furnace which operates at elevated temperatures, e.g., temperatures in excess of about 1500.degree. F., wherein the feedstock is converted into ethylene, propylene, aromatic hydrocarbons, unsaturated C.sub.4 hydrocarbons and a variety of other products. The product stream of gases (ethylene, etc.) emanating from the cracking furnace is thereafter directed through cooling means wherein the temperature of the gases is rapidly reduced. The cooled gas stream is thereafter introduced into quench prefractionation equipment for further cooling and the removal of a heavy hydrocarbon fraction from the cracked gas stream. The cracked gases are thereafter introduced into a compression zone (frequently comprising a series of four compression stages) whereby the pressure of the gas stream is increased to the pressure necessary for achieving the desired product recoveries in the product recovery zone. The pressurized cracked gas stream leaving the compression zone is then introduced into a demethanization zone to separate hydrogen and methane from the pressurized gas stream. The remainder of the pressurized gas stream is thereafter directed through a recovery zone for recovery of ethylene product from other products in the pressurized cracked gas stream.
In general, the recovery zone includes a series of fractional distillation towers which are adapted to separate the pressurized cracked gas stream into a stream of ethylene, and also streams of by-products such as propylene, crude unsaturated C.sub.4 hydrocarbons, raw pyrolysis gasoline, etc.
The crude unsaturated C.sub.4 hydrocarbon by-product stream separated in the recovery zone is usually directed to a butadiene recovery facility where high purity 1,3-butadiene (hereinafter generally referred to as simply "butadiene") is separated from the remaining C.sub.4 hydrocarbons. The remaining C.sub.4 hydrocarbons are withdrawn from the butadiene recovery facility primarily as a mixture known in the art as "butene raffinate." This mixture is generally comprised of normal butenes and isobutene. The butadiene recovery facility is generally contiguous to the ethylene production facility for integrated operation.
The recovered butadiene is an important monomer used in the production of a number of polymerization products such as synthetic rubber. It is therefore a material of significant commercial value. Butene raffinate, however, is not a desired by-product of ethylene production and is far less valuable than butadiene or ethylene. Therefore the butene raffinate is, for the most part, used as alkylation plant feed or as fuel. In view of the value of ethylene, particularly when compared to fuel gas, (the current price of ethylene is more than twice that of fuel) the inherent production of butene raffinate during ethylene production seriously detracts from the overall economics of the ethylene process.
Recycling the butene raffinate as a feedstock for the production of ethylene is not desirable due, in part, to the very low yield of ethylene that is produced by the thermal cracking of the raffinate. Indeed, this is true even if the raffinate were hydrogenated prior to cracking because of the high proportion (from 48% to 55%) of isobutane that would be present. The ultimate yield of ethylene resulting from the thermal cracking of isobutane is only about 18%. On the other hand, the isomer of isobutane, i.e., normal butane, is an excellent feedstock for the production of ethylene. Under appropriate cracking conditions, normal butane produces an ultimate yield of about 48% ethylene, which makes it an even better feedstock for the production of ethylene than gas oil or naphtha.
It would be highly desirable therefore to provide a method of converting the butene raffinate generated during ethylene production into normal butane which could be recovered or directed back to the ethylene production process, which method would be efficient, economical and readily integrated into the overall ethylene production process.
Accordingly, it is an object of the present invention to provide a process for converting a stream of unsaturated C.sub.4 hydrocarbons, which can include butadiene, into normal butane.
Another object of the invention is to provide a process for converting butene raffinate generated during the production of ethylene into normal butane which can either be recovered, or advantageously, used as one of the feedstocks for the production of ethylene to increase the overall ethylene yield of the ethylene production process.
Still another object of the invention is to provide a process for converting butene raffinate into normal butane feedstock for the production of ethylene, which process can be readily integrated into an ethylene production facility in an efficient and economical manner to increase the overall yield of ethylene.